In a condenser microphone, since the impedance of a microphone unit thereof is very high, an impedance converter such as a field effect transistor (FET) and a vacuum tube is used. If strong electromagnetic waves are applied from the outside, they are detected by the impedance converter, and hence noise sometimes occurs on an audio frequency band.
In recent years, cellular phones have come into wide use rapidly. At the time of transmission of cellular phone, very strong electromagnetic waves are radiated, so that countermeasures against electromagnetic waves have been required even in the side-entry condenser microphone used exclusively at a studio. FIGS. 3A and 3B are a front longitudinal sectional view and a side longitudinal sectional view of a side-entry condenser microphone, respectively. FIG. 4 is an exploded view of FIG. 3A.
Referring to these figures, the side-entry condenser microphone is broadly divided, in terms of structure, into a head case portion 10, a body portion 20 for supporting the head case portion 10 on the upper end side, and a connector portion 30 mounted on the lower end side of the body portion 20.
The head case portion 10 has a base ring 11 formed of a metallic material such as brass, and an upper part thereof is covered with a metallic guard net 12. Usually, a wind screen 12a formed of a permeable sheet material is additionally provided on the inner surface of the guard net 12.
In an internal space surrounded by the guard net 12, a microphone unit 13 is arranged in a state of being supported on the base ring 11 via a support bracket 14a and a shock mount material 14b having rubber elasticity. Since this microphone is of a side entry type, the sound-capturing axis of the microphone unit 13 is in a substantially horizontal direction.
The body portion 20 is formed into a cylindrical shape also using a metallic material such as brass, and a circuit board 21 is installed therein via stays 22. Although not shown in the figures, the circuit board 21 is mounted with an audio output circuit including the impedance converter, a polarization voltage generating circuit, and the like.
The connector portion 30 has a connector cover 31 integrally having a cylindrical portion 32 also formed of a metallic material such as brass, and an output connector 33 is arranged in the cylindrical portion 32. Usually, as the output connector 33, an output connector of three-pin type connected to a phantom power source via a balanced shield cable, not shown, is used.
After the microphone unit 13 and the circuit board 21, and the circuit board 21 and the output connector 33 are connected to each other by respective wiring, not shown, the head case portion 10, the body portion 20, and the connector portion 30 are connected integrally with machine screws, not shown.
Since the head case portion 10, the body portion 20, and the connector portion 30 are each formed of a metallic material, a shield case is formed by these members. However, the electrical connection between these members is due to mechanical point contact, so that the point contact portion has an impedance in terms of high frequency.
Thereupon, the conventional example has a problem in that strong electromagnetic waves generated by a cellular phone etc. intrude from the point contact portion (connecting portion of these members) having an impedance in terms of high frequency into the microphone, whereby noise is generated easily.